A highly conventional and widely used method of packaging utilizes a synthetic resinous film, which is disposed about the produce and heat sealed to enclose it therewithin. In many instances, it is desirable to open the package by peeling apart the sealed films, and this must generally be done without destroying the integrity of the film itself. Accordingly, to provide a satisfactory package of this type, a balance of properties is necessary, i.e. the seal strength must be adequate to maintain the package in a closed and protective condition, while also exhibiting release properties as will permit opening by peeling when desired.
The invention relates to blends, and a method for making the blend of a flexible film package made from films and/or laminates. Seals produced from these heat-sealable films and/or laminates are characterized by a nearly constant peel strength over an extended heat seal temperature range and by being peelable. The term "peelable" refers to a film having seal failure occur primarily at the interface of the sealing surfaces, rather than film tearing. The blends, films and/or laminates of the present invention permit the manufacture of a more consistent finished product, having a seal of predictable and constant peel strength, in spite of inevitable variations in the heat seal temperatures used in the production of such packages.
U.S. Pat. No. 4,189,519 describes one such heat sealable resin blend. This reference teaches that a resin blend comprising of at least two components, about 50 to 90 weight percent of a copolymer of about 80 to 96 weight percent of ethylene and about 4 to 20 weight percent of an ethylenically unsaturated ester, and about 50 to 10 weight percent polybutylene (preferably high molecular weight polybutylene), produces a peelable seal with a heat sealing temperature of about 160.degree.-300.degree. F., and affords a peel strength of about 0.3 to 3 pounds per square inch.
This same reference also teaches that the copolymer can contain about 12 to 14 weight percent of vinyl acetate or alternatively, the copolymer can be a graft copolymer of an unsaturated, fused ring, carboxylic acid anhydride upon high density polyethylene.
Additionally, this reference teaches that the polybutylene employed in making the heat sealable resin blend is a flexible, crystalline isotactic polymer having a density of about 0.91 and a melt index of about 0.4 to 20; most desirably, a melt index of about 1.0.
Other references teach heat sealable packaging films, such as for food stuffs which include 20 to 40 percent by weight of polybutene-1 and about 80 to 60 percent by weight of an ethylene/vinyl acetate copolymer consisting essentially of about from 5 to 28 percent vinyl acetate; see U.S. Pat. No. 3,817,812.
A variety of other references teach heat sealable films capable of forming peel seals, such as U.S. Pat. Nos. 4,550,141; 4,539,263; 4,414,053 and 3,879,492. However, none of these references appear to teach a method of blending the separate tie layer with the components of the film forming an intimate packaging film structure, without the equipment needed to dispose the tie layer adhesive on the substrate. The instant invention teaches a method of blending the separate tie layer adhesive with the components of the film such that the equipment needed to dispose the tie layer adhesive on the substrate is no longer necessary. The instant invention recognizes that conventional multi-layer peelable seal films or sheets are comprised of substrates and peelable sealants which are generally not chemically compatible and is a novel blend, and method of making a film which creates chemically compatible substrates and peelable sealants. The instant invention avoids delamination layers when the sealed layers are pulled apart and utilizes a tie adhesive to bind incompatible substrates with sealants without the need for additional equipment to bind the adhesive between the sealant and the substrate. The novel and inventive method reduces manufacturing costs for producing peelable seals.
A need has long existed for a blend which can be used to make a peelable seal which can be blended with the tie layer adhesive of the film prior to extrusion, thereby reducing the cost for making easy opening peelable seals.
The present invention which teaches a blend of polybutylene, polypropylene and modified EVA, a blend of polybutylene, polypropylene and modified low density polyethylene and a blend of polybutylene and modified low density polyethylene meets this need.